Corrugated thermoplastic sheet has proved to be unsuitable for the construction of boxes especially where relatively heavy loads are to be carried in such boxes. These relatively heavy loads may comprise fish or vegetables which weigh in the order of 4 to 15 kilograms. Boxes constructed from the corrugated thermoplastic sheet have been too weak for these loads. Such corrugated thermoplastic sheet has traditionally been 3 to 4 mm thick and has had a weight of approximately 600 grams per m.sup.2 with a buckling load resistance index of about 100. This sheet could simply be made thicker and heavier to increase its strength. However by increasing the thickness and the weight of the sheet, more material is used and consequently it is not cost effective to increase the thickness and the weight of the sheet to increase the strength of the sheet.